Apparatus and method for spatially and temporally distributing cardiac electrical stimulation

ABSTRACT

A cardiac electro-stimulatory device and method for operating same in which stimulation pulses are distributed among a plurality of electrodes fixed at different sites of the myocardium in order to reduce myocardial hypertrophy brought about by repeated pacing at a single site and/or increase myocardial contractility. In order to spatially and temporally distribute the stimulation, the pulses are delivered through a switchable pulse output configuration during a single cardiac cycle, with each configuration comprising one or more electrodes fixed to different sites in the myocardium.

FIELD OF THE INVENTION

This invention pertains to apparatus and methods for electrostimulationof the heart including cardiac pacing with an artificial pacemaker. Inparticular, the invention relates to the manner in which electricalstimulation is delivered to the heart.

BACKGROUND

Conventional cardiac pacing with implanted pacemakers involvesexcitatory electrical stimulation of the heart by an electrode inelectrical contact with the myocardium. (As the term is used herein,“excitatory stimulation” refers to stimulation intended to cause acardiac contraction.) The pacemaker is usually implanted subcutaneouslyon the patient's chest, and is connected to an electrode for each pacedheart chamber by leads threaded through the vessels of the upper venoussystem into the heart. In response to sensed electrical cardiac eventsand elapsed time intervals, the pacemaker delivers to the myocardium adepolarizing voltage pulse of sufficient magnitude and duration to causean action potential. A wave of depolarizing excitation then propagatesthrough the myocardium, resulting in a heartbeat.

The normal rhythmic impulse of the heart is first generated in pacemakertissue known as the sino-atrial (SA) node, spreads throughout the atriacausing atrial contraction, and is then conducted to theatrioventricular (AV) node where the impulse is delayed before passinginto the ventricles. The ventricles of a normal heart are thenelectrically stimulated by excitation emanating from the AV node thatspreads to the heart via specialized conduction pathways known asPurkinje fibers. These fibers lie beneath the endocardium and spreadthroughout each ventricular chamber where they penetrate into themyocardium and become continuous with the muscle fibers. The conductionvelocity of the Purkinje fibers is very rapid so that the time betweenthe impulse leaving the AV node and spreading to the entire endocardialsurface of the ventricles is only approximately 0.03 seconds. Once theimpulse has reached the ends of the Purkinje fibers, it is thentransmitted through the ventricular muscle mass by the muscle fibersthemselves with a conduction velocity only about one-sixth that of thePurkinje fibers. Because of the rapid excitation of the entireendocardial surface by the Purkinje system, however, the spread ofexcitation from the endocardial surface to the epicardial surface of theventricles takes only about another 0.03 seconds. This means that in thenormal heart, excitation of the first ventricular muscle fiber occursonly about 0.06 seconds before the last ventricular muscle fiber isexcited. The result is a synchronous contraction in which all portionsof the ventricular muscle in both ventricles begin contracting at nearlythe same time. Not only does this increase the pumping efficiency of theventricles, but it also evenly distributes ventricular wall stressduring the pumping cycle.

Unfortunately, artificial ventricular pacing with an electrode fixedinto an area of the myocardium cannot take advantage of the heart'snormal Purkinje conduction system because that system can only beentered by impulses emanating from the AV node. Thus the spread ofexcitation must proceed only via the much slower conducting ventricularmuscle fibers, resulting in the part of the ventricular myocardiumstimulated by the pacing electrode contracting well before parts of theventricle located more distally to the electrode. Although the pumpingefficiency of the heart is somewhat reduced from the optimum, mostpatients can still maintain more than adequate cardiac output withartificial pacing.

Another deleterious effect of the conduction delays brought about byartificial pacing, however, is the uneven distribution of wall stressduring the cardiac pumping cycle. The degree of tension on a heartmuscle fiber before it begins to contract is termed the preload. Becausepressure within the ventricles rises rapidly from a diastolic to asystolic value as blood is pumped out into the aorta and pulmonaryarteries, the part of the ventricle that first contracts due to a pacingpulse does so against a lower pressure preload than does a part of theventricle contracting later which unevenly stresses the myocardium. Theheart's physiological response to this uneven preload and stress iscompensatory hypertrophy in the areas of the myocardium that mustcontract against a greater pressure load. Not only can this hypertrophycause blood flow problems that may further hinder pumping efficiency,but it has been found that myocytes (i.e., cardiac muscle cells) whichare made to contract against a greater than normal mechanical load canbe induced to undergo apoptosis (i.e., genetically programmed celldeath). This may be especially true in pacemaker patients, a largeproportion of whom do not have healthy myocardium to begin with, mostcommonly as a result of ischemic heart disease.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for cardiac stimulationthat spatially and temporally distributes electrical stimulation to morethan one site of either the atria or ventricles. One embodiment involvesdistributing excitatory electrical stimulation for pacing the heart withan artificial pacemaker. A primary objective in distributing the pacingto multiple sites is to reduce disparities in the distribution ofpreload and stress during systole to thereby reduce compensatoryhypertrophy of the myocardium and/or apoptosis of myocytes. In oneembodiment, a plurality of pacing electrodes are fixed at separatesites, and a selected electrode or group of electrodes among theplurality are energized during any one cardiac cycle. The selectedelectrode or electrodes used to stimulate the heart chamber may then beswitched to a different electrode or group of electrodes on abeat-to-beat basis, at fixed time intervals, or in accordance with asensed parameter. A particular mode of practicing the invention can alsobe implemented with a conventional pacemaker having a single bipolarpacing electrode for pacing a particular heart chamber and controlcircuitry for changing the polarity of the electrode in accordance witha switching algorithm.

In another embodiment, non-excitatory electrical stimulation isdistributed over the myocardium similar to the distributed pacingdescribed above. In this case, however, rather than pacing the heart,the stimulation is employed to enhance myocardial contractility bydelivering stimulatory electrical pulses synchronized with intrinsiccardiac activity and during the myocardial refractory period. Suchstimulation may be delivered either by a specialized electro-stimulatorydevice or a pacemaker configured to deliver non-excitatory stimulationto the heart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary electro-stimulatory cardiacdevice for practicing the present invention.

FIG. 2 shows a bipolar electrode.

DESCRIPTION OF THE INVENTION

In accordance with the invention, an electro-stimulatory devicespatially and temporally distributes stimulatory pulses to multiplesites of either the atria or ventricles by switching between differentelectrode configurations. Such electro-stimulatory devices include bothdevices for pacing the heart with excitatory stimulation (i.e.,artificial pacemakers) as well as devices for delivering non-excitatorystimulation pulses to the heart. Furthermore, the term should be takento include any cardiac rhythm management device, such as an implantablecardioverter/defibrillator, with a pacing or non-excitatory stimulationfunction incorporated therein.

Pacemakers have been constructed for delivering pacing pulses tomultiple ventricular or atrial sites, including so-called biventricularpacemakers where pacing pulses are delivered to both ventricles byseparate electrodes during a cardiac cycle. (See, e.g., U.S. Pat. Nos.5,792,203 and 4,928,688, referred to herein as the '203 and '688patents, which are hereby incorporated by reference.) One type ofmulti-site pacing involves fixing two or more pacing electrodes toseparate sites of the same heart chamber, either an atrium or aventricle. For example, one electrode may be fixed to the apical regionof either the right or left ventricle with the other electrode fixed toa basal region of the same ventricle. In the case of the left ventricle,this may be most easily accomplished by using a coronary sinus lead (SeeU.S. Pat. No. 5,935,160, hereby incorporated by reference) with distaland proximal electrodes. The ventricle can be paced in accordance with aprogrammed pacing mode with the electrodes being energizedsimultaneously during each pacing output in order to achieve nearsimultaneous activation of the ventricle. Alternatively, the pacingstimuli can be delivered to the ventricular electrodes sequentially witha specified time delay in order to take into account differingconduction times within the ventricle.

Biventricular pacemakers, such as described in the '688 patent, providepacing to both ventricles in the presence of an interventricularconduction defect. Biventricular pacing may also be useful in certaininstances, however, when interventricular conduction pathways areintact. For example, a biventricular pacemaker of the kind described inthe '688 patent can be configured with one electrode pacing theposterior or lateral wall of the left ventricle and the other electrodepacing the apex of the right ventricle. When only the left ventricularsite is paced, the spread of activation is from the paced site as wellas from the part of the normal left ventricular conduction system thatis still intact (which would be in the vicinity of the basal region ofthe interventricular septum). Thus, although only one site is paced, thespread of activation is from two sites. If, on the other hand, bothsites are paced, either simultaneously or with a specifiedinterventricular delay between the right and left ventricles, a similarspread of activation occurs from the left ventricular site, but achanged activation occurs from the right ventricular site that breaksthrough into the left ventricle somewhere in the apical region of theinterventricular septum. There is thus left ventricular activationspreading from two (or more) sites which is different from theactivation that occurs with left ventricular pacing alone. By adjustingthe interventricular delay, the relative spread of activation betweenthe two pacing sites can be modified which thereby affects the wallstress developed near these sites and throughout the ventricle duringcontraction.

Multi-site pacemakers such as described in the '688 and '203 patents arecapable of delivering pacing stimuli to multiple sites of the atria orventricles in which the multiple sites are all stimulated during any onecardiac cycle. Each of the pulses delivered to the multiple pacing sitesduring the cardiac cycle, however, must be of sufficient energy (i.e.,sufficient pulse amplitude and duration) to cause heart musclecontraction were it to be delivered alone, since pacing stimuli appliedover a wide area cannot sum together. Both because of increased batteryrequirements and the physiological effects on the heart, therefore, itmay not be desirable to deliver simultaneous pacing at multiple sites.Furthermore, certain patients may benefit from activating parts of theventricle at different times in order to distribute the pumping load todifferent areas of the ventricle. For example, a relatively weak area ofthe myocardium could be activated before stronger areas to result in adistribution of wall stress that increases pumping efficiency. In any ofthe situations discussed above, therefore, it may be desirable tospatially and temporally distribute the pacing. In accordance with theinvention, a multi-site pacemaker accomplishes this objective byswitching the output of pacing pulses between selected electrodes orgroups of electrodes during different cardiac cycles.

Another useful application of spatial/temporal switching is todistribute the effects of localized multi-site stimulation pulses thatcan improve myocardial contractility. (See Braunwald et al., Sustainedpaired electrical stimuli; Slowing of the ventricular rate andaugmentation of contractile force, American Journal of Cardiology14:385, 1964; Sabbah, et al., Delivery of Non-excitatoryContractility-Modulation Electric Signals Improve Left VentricularPerformance in Dogs with Heart Failure, Circulation Supplement 1, Vol.100, pg. I-122, Abstract 631, Nov. 2, 1999). In this type of pacing,multiple pulses are output to a pacing electrode during one or morephases of the action potential in order to improve local contractilefunction. For example, a non-excitatory stimulating pulse could beapplied after a pacing pulse and during the absolute refractory periodto increase intracellular calcium concentration which thereby enhancescontractility. Because this type of stimulation also increases localoxygen consumption, switching the stimulation site by spatially andtemporally distributing the stimulation in accordance with the presentinvention serves to help prevent deleterious effects at the pacing site.Such effects may be due to biological adaptation resulting fromexpression of intracellular proteins involved in ion transport andcontractile function.

Cardiac pacemakers stimulate the heart by delivering a voltage pulse toan electrode situated on or in the myocardium, with the stimulatingelectrode usually acting as a cathode. The electrode may be “fixed” to aheart chamber in one of three basic ways: it may be left to float insideone of the heart chambers near the heart wall, actively fixed in themyocardium by implantation, or passively fixed against the endocardiumvia a tissue encapsulation reaction. In order to pace the heart, theelectrode must deliver a voltage pulse of sufficient magnitude andduration to cause a propagated action potential resulting in acontraction of the myocardium (i.e., an excitatory stimulation). Inaccordance with the invention, pacing is delivered to a heart chamberthrough a switchable configuration of pacing electrodes, wherein a pulseoutput configuration is defined as a specific subset of a plurality ofelectrodes fixed to the paced chamber and to which pacing pulses areapplied. A plurality of different pulse output configurations may bedefined as subsets of electrodes that can be selected for pacing. Byswitching the pulse output configuration to a different configuration,pacing to the heart chamber is thereby temporally distributed among thetotal number of fixed electrodes. For example, two electrodes may befixed to a heart chamber with each electrode constituting a pulse outputconfiguration, and switching the pulse output configuration alternatelybetween the electrodes.

In various embodiments of the invention, a particularelectro-stimulatory configuration may comprise one or more electrodes,and a particular electrode may belong to one or more configurations. Aplurality of different pulse output configurations may then be definedas subsets of the total number of stimulatory (for pacing ornon-excitatory stimulation) electrodes fixed to the paced heart chamber.For example, the pacemaker may have N ventricular stimulatory electrodeswith a particular subset n of those electrodes selected for pacing in aparticular pulse output configuration, so that stimuli are applied to ndifferent sites in the ventricle during a cardiac cycle. In accordancewith a particular switching algorithm, the pulse output configurationmay then be switched to a different subset m of the N channels tospatially and temporally distribute the stimulation. The subset m makingup the configuration that is switched to may be any combination of the Nelectrodes. That is, the number of electrodes in the subset m may be thesame or different as the number of electrodes in the subset n, and thetwo subsets may have electrodes in common. A pulse output configurationmay also be further defined as a particular temporal sequence of pacingor non-excitatory stimulation pulses applied to the electrodes making upthe configuration. For example, a pulse output configuration may consistof two electrodes fixed to separate sites of a heart chamber, and theheart chamber is paced (or otherwise stimulated) by applying a pulse toone electrode and then the other after a specified time delay.

The present invention may be applied to distribute the pacing ornon-excitatory stimulation of a heart chamber where the stimulatedchamber may be either the atria or ventricles, or both. Furthermore,since the atria and ventricles both consist of paired chambers (i.e.,right/left ventricle or atrium), the electrodes may be fixed to thestimulated chamber in a number of different ways. For example, in oneembodiment, a ventricular pacemaker (or other electro-stimulatorydevice) has a set of pulse output configurations with each configurationcomprising electrodes fixed to a single ventricle. In anotherembodiment, the available configurations may comprise electrodes fixedto both ventricles. In still another embodiment, each configuration isconfined to one ventricle, but configurations are available for eachventricle so that stimulation pulses can be applied to either ventricle.

Pacing or non-excitatory stimulation may be distributed over themyocardium of a heart chamber by switching between pulse outputconfigurations according to a switching algorithm. Switching betweenpulse output configurations can be made to occur on a beat-to-beat basisso that the stimulated site is alternated on every cardiac cycle ormultiples of cycles. Alternatively, the configurations can be switchedat periodic time intervals so that the pacing site is changed eitherrapidly (e.g., every minute) or over a much longer time period (e.g.,weekly, monthly, etc.) depending upon the needs of the patient. Theswitching may also performed in accordance with a sensed physiologicalparameter so that the pulse output configuration is switched when thevariable falls outside of a specified range. In one embodiment, cardiacoutput (as determined by, e.g., impedance measurement as described inU.S. Pat. No. 4,674,518 issued to Salo et al., the disclosure of whichis hereby incorporated by reference) is monitored, and the pulse outputconfiguration is switched if it falls below a specified value. Inanother implementation, the depolarization following a pacing pulse issensed at one or more sites at some distance from the pacing site orsites, and the time delay between the pacing pulse and the senseddepolarization is measured. The pulse output configuration is thenswitched if the time delay exceeds a specified value. In anotherembodiment, the mean or minimum heart rate is monitored (e.g., for a 24hour period) and the pulse output configuration is switched if the rateexceeds a threshold value. The specified parameter values used forswitching the pulse output configuration is preferably programmable tosuit the needs of individual patients. The change in the sensedphysiological parameter that results when a particular pulse outputconfiguration is switched on may also be used to select the best pulseoutput configuration. The device may be programmed to go through atesting sequence in which pulse output configurations are alternatelyswitched on and off, with the pulse output configuration then selectedin accordance with which one produces the greatest improvement in thesensed parameter. For example, the pulse output configuration thatproduces the highest cardiac output, the shortest measured time delay,or the lowest mean or minimum heart rate may be selected for pacing.Lastly, it may be desirable in some patients to simply configure thepacemaker to use only one pulse output configuration and switch toanother configuration at a discretionary time using a telemetryinterface.

A block diagram of a cardiac electro-stimulatory device having twoventricular stimulation channels is shown in FIG. 1. The control unit ofthe pacemaker is made up of a microprocessor 10 communicating with amemory 12 via a bidirectional data bus [13], where the memory 12typically comprises a ROM (read-only memory) for program storage and aRAM (random-access memory) for data storage. The control unit could alsoinclude dedicated circuitry either instead of, or in addition to, theprogrammed microprocessor for controlling the operation of the device.The control unit is capable of operating the device in a number ofprogrammed modes where a programmed mode defines how pulses are outputin response to sensed events and expiration of time intervals. Atelemetry interface 40 is also provided for communicating with anexternal programmer.

The pacemaker has atrial sensing/stimulation channels comprisingelectrode 34, lead 33, sensing amplifier 31, pulse generator 32, and anatrial channel interface 30 which communicates bidirectionally with aport of microprocessor 10. The device also has two ventricularsensing/stimulation channels that include electrodes 24 a-b, leads 23a-b, sensing amplifiers 21 a-b, pulse generators 22 a-b, and ventricularchannel interfaces 20 a-b where “a” designates one ventricular channeland “b” designates the other. For each channel, the same lead andelectrode are used for both sensing and stimulation. The channelinterfaces 20 a-b and 30 include analog-to-digital converters fordigitizing sensing signal inputs from the sensing amplifiers andregisters which can be written to by the microprocessor in order tooutput stimulation pulses, change the stimulation pulse amplitude, andadjust the gain and threshold values for the sensing amplifiers.

The device of FIG. 1 can be configured for practicing the presentinvention in one embodiment by programming its microprocessor to useonly one ventricular stimulation channel during any one cardiac cycle,with the ventricular electrodes 24 a and 24 b fixed at either separatesites in one ventricle or fixed at sites in each ventricle. A selectedstimulation channel thus constitutes a pulse output configuration thatcan be switched by the microprocessor according to a switching algorithmas described above. By switching between the stimulation channels, thestimulation is spatially and temporally distributed over the myocardium.The device as shown has two separate ventricular stimulation channelswith separate pulse generators and electrodes for each channel. Thus,the selection of a stimulation channel is effected simply by themicroprocessor commanding one or the other of the channels to deliver apulse. In another embodiment, the two electrodes 24 a-b could beconnected to a common pulse generator through switching circuitry thatswitches from one electrode to another in accordance with an output fromthe microprocessor. Also, instead of separate ventricular stimulationelectrodes 24 a-b, a single bipolar electrode 50 as shown in FIG. 2could be used, with the device having circuitry for switching thepolarity of voltage pulses output to the electrodes so that either thetip 51 or ring 52 becomes the cathode. Thus if the electrode 50 ispassed into the right ventricle with the tip 51 fixed at the apex andthe ring 52 residing at the base, switching the polarity of the voltagepulses enables either the apex or base to be stimulated. Similarly, ifthe electrode 50 is passed into the left ventricle with the ring 52residing in the coronary sinus and the tip 51 fixed in the myocardiumthrough a cardiac vein, both basal and apical regions of the leftventricle can be stimulated.

Although the invention has been described in conjunction with theforegoing specific embodiments, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Other such alternatives, variations, and modifications are intended tofall within the scope of the following appended claims.

What is claimed is:
 1. A cardiac electro-stimulatory device forspatially and temporally distributing stimulation pulses to a heartchamber, comprising: a plurality of electrodes adapted for fixation to aheart chamber; a plurality of defined pulse output configurations,wherein each pulse output configuration constitutes a subset of theplurality of the fixed electrodes; and, a control unit and pulsegenerating circuitry for outputting non-excitatory stimulation pulses toa selected one of the plurality of pulse output configurations in orderto stimulate the heart chamber during a refractory period, wherein thepulse generating circuitry includes a separate pulse generator for eachelectrode of the selected pulse output configuration.
 2. The device ofclaim 1 wherein the selected pulse output configuration is switched toanother configuration in accordance with a switching algorithm executedby the control unit.
 3. The device of claim 2 wherein the control unitis programmed to switch the pulse output configuration on a beat-to-beatbasis.
 4. The device of claim 2 wherein the control unit is programmedto switch the pulse output configuration after a specified number ofheartbeats.
 5. The device of claim 2 wherein the control unit isprogrammed to switch the pulse output configuration at a specified timeinterval.
 6. The device of claim 2 wherein the control unit isprogrammed to switch the pulse output configuration in accordance with asensed parameter.
 7. The device of claim 6 wherein the sensed parameteris related to cardiac output.
 8. The device of claim 6 wherein thesensed parameter is a sensed time delay of a depolarization occurring inan area of the myocardium after delivery of a pacing pulse.
 9. Thedevice of claim 6 wherein the sensed parameter is the mean or minimumheart rate measured over a time interval.
 10. The device of claim 1wherein each pulse output configuration comprises one or more electrodesfixed to the heart chamber.
 11. The device of claim 1 further comprisinga telemetry interface and wherein the control unit is programmed toselect one of the plurality of pulse output configurations in responseto commands received from the telemetry interface.
 12. The device ofclaim 1 wherein a pulse output configuration is further defined as aparticular temporal sequence of pulses output to the electrodes makingup the configuration.
 13. The device of claim 1 wherein the plurality ofelectrodes comprises a bipolar electrode and further comprisingcircuitry for switching the polarity of pulses output to the electrode,each selected polarity thus constituting a pulse output configuration.14. The device of claim 1 further comprising one or more pulsegenerators for outputting pacing pulses in conjunction with thenon-excitatory stimulation pulses in accordance with a programmed pacingmode.
 15. A method for operating a cardiac pacing device, comprising:outputting pacing pulses to a selected subset of a plurality of pacingelectrodes in accordance with a programmed pacing mode, wherein thesubset of electrodes to which pulses are output is defined by a pulseoutput configuration; and, switching the pulse output configurationutilized for one or more cardiac cycles to another pulse outputconfiguration for one or more subsequent cardiac cycles in accordancewith a sensed time delay of a depolarization occurring in an area of themyocardium after delivery of a pacing pulse.
 16. The method of claim 15further comprising outputting non-excitatory stimulation pulses toselected electrodes in conjunction with pacing pulses.
 17. The method ofclaim 16 further comprising switching the pulse output configuration inaccordance with a sensed time delay of a depolarization occurring in anarea of the myocardium after delivery of a pacing pulse.
 18. The methodof claim 15 wherein the switching algorithm comprises switching thepulse output configuration on a beat-to-beat basis.
 19. The method ofclaim 15 wherein the switching algorithm comprises switching the pulseoutput configuration at a specified time interval.
 20. The method ofclaim 15 wherein the switching algorithm comprises switching the pulseoutput configuration in response to a command received by the devicethrough a telemetry interface.
 21. The method of claim 15 furthercomprising switching the pulse output configuration in accordance withthe mean or minimum heart rate measured over a time interval.
 22. Themethod of claim 15 wherein the switching algorithm comprises switchingthe pulse output configuration after a specified number of heart beats.23. The method of claim 15 wherein a pulse output configuration isfurther defined as a temporal sequence of pulses output to the fixedelectrodes during a cardiac cycle.
 24. The method of claim 15 whereinthe stimulation pulses are output to a bipolar electrode having tip andring electrodes fixed to a heart chamber, and further comprisingswitching the pulse output configuration by alternating the polarity ofthe delivered pulses.
 25. The method of claim 24 wherein the bipolarelectrode is passed through the coronary sinus of a patient such thatthe tip of the electrode is fixed to the apical region of the leftventricle while the ring is in electrical contact with a more superiorregion of the left ventricle.
 26. The method of claim 24 wherein thebipolar electrode is passed into the right ventricle of a patient suchthat the tip of the electrode is fixed to the apical region of theventricle while the ring is in electrical contact with a more superiorregion of the ventricle.
 27. A cardiac pacemaker for spatially andtemporally distributing pacing pulses, comprising: a plurality ofelectrodes adapted for disposition near a heart chamber; a plurality ofdefined pulse output configurations, wherein each pulse outputconfiguration constitutes a subset of the plurality of the electrodes; acontrol unit and pulse generating circuitry for outputting pacing pulsesto a selected one of the plurality of pulse output configurations inaccordance with a programmed mode; wherein the control unit isprogrammed to utilize a first pulse output configuration during one ormore cardiac cycles and switch to a second pulse output configurationduring one or more subsequent cardiac cycles in accordance with a sensedtime delay of a depolarization occurring in an area of the myocardiumafter delivery of a pacing pulse.
 28. The pacemaker of claim 27 whereineach pulse output configuration comprises one or more electrodes fixedto the heart chamber.
 29. The pacemaker of claim 27 wherein a pulseoutput configuration is further defined as a particular temporalsequence of pulses output to the electrodes making up the configuration.30. The pacemaker of claim 27 wherein the plurality of electrodescomprises a bipolar electrode and further comprising circuitry forswitching the polarity of pulses output to the electrode, each selectedpolarity thus constituting a pulse output configuration.
 31. Thepacemaker of claim 27 further comprising separate pulse generators foreach electrode.
 32. A cardiac pacemaker for spatially and temporallydistributing pacing pulses, comprising: a plurality of electrodesadapted for disposition near a heart chamber; a plurality of definedpulse output configurations, wherein each pulse output configurationconstitutes a subset of the plurality of electrodes; a control unit andpulse generating circuitry for outputting stimulation pulses to aselected one of the plurality of pulse output configurations in responseto sensed cardiac activity and lapsed time intervals in accordance witha programmed mode; wherein the control unit is programmed to switch thepulse output configuration in accordance with the mean or minimum heartrate measured over a time interval.
 33. The pacemaker of claim 32wherein each pulse output configuration comprises one or more electrodesfixed to a heart chamber.
 34. The pacemaker of claim 32 wherein a pulseoutput configuration is further defined as a particular temporalsequence of pulses output to the electrodes making up the configuration.35. The pacemaker of claim 32 wherein the plurality of electrodescomprises a bipolar electrode and further comprising circuitry forswitching the polarity of pulses output to the electrode, each selectedpolarity thus constituting a pulse output configuration.
 36. Thepacemaker of claim 32 further comprising separate pulse generators foreach electrode.